Impact of trace extrinsic defect formation on the local symmetry transition in spinel LiNi0.5Mn1.5O4−δ systems and their electrochemical characteristics

Abstract

Many fundamental studies have been conducted on the electrochemical and electronic structures in transition metal cation-substituted LiNi_0.5Mn_1.5O₄ systems. These systems have potential use as 5 V-level high voltage cathode materials for lithium ion batteries, but there are only a few reports regarding the control of their symmetry transitions which contribute the electronic structures and Li⁺ transport efficiency. We address this solid chemistry and the corresponding electrochemical characteristics using both systematic experimental and theoretical approaches. Trace substitution of Cu²⁺ with Mn⁴⁺ (Cu_Mn) can promote the symmetry transition from Fdm to P4₃32 phase in oxygen-deficient LiNi_0.5Mn_1.5O_4−δ. This behavior is detectable by Fourier-transform infrared and Raman spectroscopies but undetectable by X-ray diffraction, suggesting that the symmetry transition was localized in the space near the point of extrinsic defects Cu_Mn in the spinel framework. Notably, a very small amount of Cu²⁺ substitution not only affects the local atomic arrangement but also remarkably affects the macroscopic electrochemical redox responsiveness, including the inactivation of Mn³⁺/Mn⁴⁺ redox couples, suppression of Mn ion dissolution, and enhancing the C rate capability (increasing the electron conductivity, and reducing the activation barrier for lithium ion hopping along the most energetically preferable 8a–16c–8a route) and cyclability.